How is space for firmware in internal memory managed?

Basically, I'm looking for an older (aka cheap) android t-mobile phone with qwerty. The main issue, aside from slow processor/low ram seems to be actual space for apps/phone stuff/etc.

My question is, if i get a phone with 512MB internal, and root/use a small-footprint firmware, how much space will be left in the internal storage for all MY data? Is there a fixed amount of space set aside for firmware (that doesn't sound right)?

I had a cheap Archos tablet awhile back and learned quickly to use app2sd, which I'm fine with. I really just don't know how much internal storage is ENOUGH for me, which is obviously relative. Let me outline the main things I'm looking to do:

I'm not a power user by any means, but I need to be able to accomplish those tasks without constantly hitting low memory or waiting minutes for load time. Definitely have enough know-how to figure out rooting.

Any advice would be great, especially example footprints of common firmwares/apps/etc.

If you wish to play a few games and have some nice apps, 512MB internal is REALLY pushing it. Most of the time, around 200-500MB storage will be put aside for the OS alone, and then additional space for apps and games. But, as long as the phone that you purchase has a microSD slot (most older android phones do), you can always expand the memory with that and use apps to SD to preserve your internal memory.

I'm trying to stay around $100. I've been looking on swappa and was looking at a G2 (I'm on T-mobile). The G2 has 4g internal I believe, but the one I was looking at before, the Samsung Gravity Smart (I think) had only 512mb.

So basically, there isn't a set amount of space reserved for firmware, it's just however much space the firmware happens to take. So I could root and use a smaller firmware and have some more space (if I go with a smaller size).